1. Field of the Invention
This invention relates to detector circuits, and more particularly to circuits for detecting the presence or absence of current having a varying amplitude, or specifically alternating current, flowing in an external circuit, and for generating a digital signal in response thereto.
In situations in which the presence or absence of, in particular, an alternating current flowing in one electrical circuit is indicative of the operational condition of that circuit, it is often desirable to provide a current detection circuit that can give a rapid indication of the interruption of current to the first circuit. The indication of such an interruption may be utilized, for example, to actuate a second circuit having an effect opposing that of the first circuit. The current detector may be effectively used to determine that the first circuit, which may actuate a control load such as a motor, has switched itself off and cleared itself before the actuation of, e.g., a motor brake circuit.
2. Description of the Prior Art
Heretofore, alternating current detection systems have presented several problems. For example, the circuits responded relatively slowly to changes in the state of the a-c voltage applied thereto, requiring several cycles to indicate whether an external circuit had cleared. Another problem was that the prior art circuits themselves imposed a large load on the external circuit, wasting energy and bleeding power therefrom. Still another problem was that prior art circuits have generally had analog outputs rather than digital outputs. As a result, some means must be provided to avoid an erroneous indication that alternating current in the external circuit has been interrupted when the current passes through zero during the normal change in polarity associated with alternating currents. U.S. Pat. No. 3,725,795, issued to L. 0. Hesenhimer on Apr. 3, 1973, utilizes an R-C network to obviate this problem; however such circuits generally require a time delay after current interruption before the detection circuit indicates that current in the external circuit has switched off. Furthermore, because the prior art devices have analog outputs, they are not directly and immediately compatible with digital switching circuitry.